Well we're here!
Further to the above comment, when we got to T2 check in and presented our passports, 84kgs of baggage and said printout, we were told that Singapore air only offer an extra 10kgs to those holding resident visas to AUSTRALIA!! Seems odd and rather discriminatory. However, since we had written proof that the extra had been offered to us as NZ visa holders they honoured this, so we got 40kgs each, then the extra 4kgs were waved through.
NB we also had to weigh our cabin bags, these have a 10kg limit. Eek! Pockets were filled, bags weighed, then pockets emptied back into bags.
Check in overall was highly laborious, it's all automated at T2, but the machine which asks you to scan your visa can't scan visas! - just go to a manned queue.
Unless you're flying business class when you automatically get to check in with a human. Thanks Heathrow for making economy passengers feel even more second class.
So if you get written confirmation of extra baggage I would go for it, but take the proof with you.